1. Field of the Invention
The present invention relates to a glass bulb used in a cathode ray tube, and particularly to a glass panel constituting a front surface portion of the glass bulb.
2. Description of the Related Art
As shown in FIG. 3, a glass bulb 1 used in a cathode ray tube generally comprises a glass panel 10 serving as a front surface portion, a funnel 20 serving as a back structural member and a neck 30 for mounting an electron gun therein. The glass panel 10 includes a face portion 11 of substantially rectangular shape having an effective screen for displaying an image, and a skirt portion 13 continued from the periphery of the face portion 11 via a blend R portion 12 and having a seal edge surface 14 for sealing with the funnel 20. In the case of a color cathode ray tube, the glass panel 10 is sealed between the seal edge surface 14 of the skirt portion 13 and a seal edge surface of the funnel 20 via frit glass and the like.
Since the glass bulb 1 for cathode ray tube is used as a vacuum vessel of which interior is evacuated to vacuum, a stress caused by a pressure difference between inside and outside of the bulb will act on the outer surface of the glass bulb 1. In the case of the glass bulb 1 which is not a spherical shell, however, as shown in FIG. 4, there arises complicated distribution of stress such that an area of tensile stress denoted by arrows toward outside of the bulb and an area of compressive stress denoted by arrows toward inside of the bulb are present at the same time.
The vacuum tensile stress generated in the glass bulb 1 usually becomes maximum in the area ranging from an end of the face portion on the short axis of the glass panel 10 to the skirt portion, and when a mechanical or thermal shock exceeding a certain degree is applied to the glass bulb 1 from the outside, the glass bulb 1 is broken from the vicinity of the region where the maximum vacuum tensile stress is generated, that is the area ranging from the end of the face portion 11 to the skirt portion 13 as its origin of breakage, resulting in implosion. Therefore, the glass bulb 1 used in a cathode ray tube is usually designed to have an enough mechanical strength to suppress the vacuum tensile stress to a predetermined value or less.
Though the distribution of vacuum tensile stress depends on the size and shape of the glass bulb, the design of shape, wall thickness and the like is made so as to suppress the vacuum tensile stress generated at the seal edge portion between the glass panel and the funnel to less than 8.4 MPa, which is one standard of mechanical strength required for a glass bulb determined in consideration of safety factors such as shocks applied from the outside.
For this reason, in conventional glass panels for cathode ray tube, in order to suppress the vacuum tensile stress to less than the predetermined value while retaining the mechanical strength at the time of being used for a glass bulb, such measures have been taken as increasing the wall thickness of the glass, elongating the skirt portion for relieving and distributing the vacuum tensile stress generated in the vicinity of the skirt portion to thereby reduce the peak value thereof, and the like.
In conventional glass panels for cathode ray tube, however, since the weight of the glass is increased because of the increased wall thickness of the glass or the elongated skirt potion, there is a problem that the glass panel is inferior in operability and workability. In particular, as for the glass panel whose skirt portion is elongated, since the glass panel immediately after forming has not been sufficiently solidified, there arises a problem that the skirt portion tends to incline toward inside or outside and thus the glass panel is easy to deform.
In view of the above, it is an object of the present invention to provide a glass panel for cathode ray tube of large size having a high flatness of a face portion, which realizes weight reduction by shortening a skirt portion while retaining the predetermined mechanical strength as a glass bulb, as well as suppresses deformation immediately after forming.
The present invention was accomplished for solving the above-mentioned problems, by using various sizes of glass panels for cathode ray tube and measuring a panel weight and maximum vacuum tensile stress when used as a glass bulb for a plurality of samples having different skirt lengths and glass wall thicknesses of seal edge surface.
That is, a glass panel for cathode ray tube according to the present invention comprises a face portion of substantially rectangular shape, and a skirt portion connected with a periphery of the face portion via a blend R portion and having a seal edge surface for sealing with a funnel, and is characterized in that an effective screen diameter D(mm) of the glass panel along a diagonal axis thereof is in the range of 500xe2x89xa6D less than 650; an average radius of curvature of an outer surface of the face portion is more than or equal to 10,000 mm in any radial direction passing the center of the face portion; and a distance h(mm) along a tube axis from a contact between an effective screen end of an inner surface of the glass panel and the blend R portion to the seal edge surface at least in a short axis of the glass panel, and a glass wall thickness t (mm) of the seal edge surface satisfy the relationships of: 0.07Dxe2x89xa6hxe2x89xa60.11D, 0.015Dxe2x89xa6txe2x89xa60.025D and (D/25.4)2xe2x89xa6txc3x97hxe2x89xa6(D/25.4+3)2.
Furthermore, a glass panel for cathode ray tube according to the present invention is characterized in that an effective screen diameter D(mm) of the glass panel along a diagonal axis thereof is more than or equal to 650; an average radius of curvature of an outer surface of the face portion is more than or equal to 10,000 mm in any radial direction passing the center of the face portion; and a distance h(mm) along a tube axis from a contact between an effective screen end of an inner surface of the glass panel and the blend R portion to the seal edge surface at least in a short axis of the glass panel, and a glass wall thickness t (mm) of the seal edge surface satisfy the relationships of: 0.08Dxe2x89xa6hxe2x89xa60.11D, 0.015Dxe2x89xa6txe2x89xa60.020D and (D/25.4)2xe2x89xa6txc3x97hxe2x89xa6(D/25.4+2.5)2.
According to the present invention, in a glass panel for cathode ray tube having a large size with an effective screen diameter D along the diagonal axis of the glass panel of more than or equal to 500 mm, and a high flatness with an average radius of curvature of the outer surface of a face portion of more than or equal to 10,000 mm, when a distance h along a tube axis from a contact between an effective screen end of an inner surface of the glass panel and a blend R portion to a seal edge surface is defined as a length of a skirt portion, and a glass wall thickness of the seal edge surface of the skirt portion is defined as t, ranges of h and t are specified by ratios with respect to the effective screen diameter D which is a substantial size of the glass panel, and a product of h and t is also specified to satisfy a predetermined range in relation to the effective screen diameter D, from the view point of mechanical strength and weight reduction. In this way, deformation of the skirt portion due to inclination is suppressed, reduction in mechanical strength due to shortening of the skirt portion is compensated by the glass wall thickness of the seal edge surface, and thus weight reduction of the glass panel is accomplished while retaining the predetermined mechanical strength.
The reason why these specifications were made on the short axis of the glass panel is that the maximum vacuum tensile stress on the glass bulb is usually generated in the region ranging from the end of the face portion to the skirt portion on the short axis of the glass panel.
In the case of a glass panel for cathode ray tube having an effective screen diameter D (mm) of 500xe2x89xa6D less than 650, when the length h of the skirt portion and the glass wall thickness t of the seal edge surface of the skirt portion are set in the ranges of 0.07D greater than h and/or 0.015D greater than t, or txc3x97h less than (D/25.4)2, the skirt portion becomes too short and the wall thickness t of the seal edge portion is too small, causing the vacuum tensile stress value of the seal edge portion generated by evacuation of the glass bulb to become more than 8.4 MPa, so that it is impossible to obtain the desirable mechanical strength required for a glass bulb.
On the other hand, when h greater than 0.11D, the length of the skirt portion cannot be shortened, so that weight reduction of the glass panel cannot be accomplished and the skirt portion is likely to deform by inclination immediately after forming of the glass panel. Further, when t greater than 0.025D or (D/25.4+3)2 less than txc3x97h, it is impossible to reduce the weight of the glass panel.
In the case of a glass panel for cathode ray tube having an effective screen diameter D (mm) of more than or equal to 650, when the length h of the skirt portion and the glass wall thickness t of the seal edge surface of the skirt portion are set in the ranges of 0.08D greater than h and/or 0.015D greater than t, or txc3x97h less than (D/25.4)2, the skirt portion becomes too short and the wall thickness of the seal edge portion is too small, causing the vacuum tensile stress value of the seal edge portion generated by evacuation of the glass bulb to become more than 8.4 MPa, so that it is impossible to obtain the desirable mechanical strength required for a glass bulb.
On the other hand, when h greater than 0.11D, the length of the skirt portion cannot be shortened, so that weight reduction of the glass panel cannot be accomplished and the skirt portion is likely to deform by inclination immediately after forming of the glass panel. Further, when t greater than 0.020D or (D/25.4+2.5)2xe2x89xa6txc3x97h, it is impossible to reduce the weight of the glass panel.